In Firefox type about:config in the address bar and press ENTER.
Search for Keyword
Locate and double-click the entry for keyword.URL
Modify it by add the below set of values :
Yahoo: http://search.yahoo.com/search?p=
Ask: http://www.ask.com/web?q=
Bing: http://www.bing.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=
Google: http://www.google.com/search?&q=